1. Field of the Invention
The invention relates to new noble metal preparations for producing noble metal decorations on decoratively fireable substrates such as glass, ceramic, bone china and porcelain, and a process for their preparation. The preparations are suitable. for direct printing and indirect printing, which is why the invention relates to both use of the noble metal preparation and to a transfer containing such a noble metal preparation.
2. Description of Related Art
Various noble metal preparations, in particular gold preparations, which can be applied to decoratively fireable substrates using direct or indirect printing, that is using a transfer technique, are already known and lead to a bright or burnishable noble metal decoration in the decoration firing process. Preparations which lead to bright decorations are called bright gold preparations. Bright gold preparations contain one or more organic noble metal compounds which are substantially soluble in an organic medium. Typical noble metal compounds for bright gold preparations are so-called resinates, in particular however sulforesinates, that is thiolates of terpenethiols, and synthetic noble metal thiolates of various other organic mercapto compounds. So-called burnished noble metal preparations, in particular burnished gold preparations, contain the noble metal in the form of compounds which are soluble and/or insoluble in the medium such as, for example, gold sulfide and many elemental noble metals in powder form. These preparations usually also contain one or more soluble fluxes, in particular organic compounds, such as resinates, carboxylates and alcoholates, in particular of elements from the group comprising Rh, Bi, Cr, V, Ni, Co, Fe, Sn, Zr, Ta, Si, B and Al, which have an effect on the structure of the noble metal film and its adhesion to the substrate. A medium which is also suitable for use in preparations for transfers substantially consists of one or more binders which are mutually compatible and solvents for the resins and noble metal compounds, or it may be largely solvent-free and substantially consist of thermoplastic resins which are liquid at the application temperature and optionally wax-like compounds. Reference is made, by way of example, to U.S. Pat. Nos. 3,092,504 and 3,163,665, DE-AS 14 21 865 and EP-B 0 440 877.
In the case of transfers required for indirect printing, an image layer consisting of the noble metal preparation or containing this is applied directly to a support, in particular paper provided with a water-soluble size layer or a wax layer or onto a base layer and this layer is overfilmed directly or after applying an intermediate layer containing a glass flux. Reference is made, by way of example, to DE-AS 12 32 168, DE 36 16 547 C1 and DE 31 22 546 C2.
Although the binders contained in the medium in noble metal preparations burn away during the firing process, they have a considerable effect on the quality of the decoration. It has been shown that dark decorations, decorations with streaks in the gloss and/or insufficient burn-out and with inadequate adhesion may often be produced, depending on the binder system used. Another reason for defects in decorations may comprise soluble noble metal compounds being dissolved out of the image layer during the overfilming process, which leads to reduced brilliance, red fringes and to the production of streaks and/or poorly defined decorations. In the case of transfers, decoration defects result from tears.
To solve or reduce the problems mentioned above, various processes have been suggested. According to DE-AS 12 32 168, an impermeable protective layer for the covering layer, made of an oxidisable or polymerisable, alkyd, phenol, urea or epoxy resin material or a material containing dry oil, is placed between the image layer and the covering layer. The additional layer reduces fringes round the decoration but increases the cost.
The image layer in the transfer according to DE-PS 36 16 547 is based on a bright noble metal preparation which contains 8 to 30 wt. % of a thermoplastic resin. A resin from the group comprising asphalt resin, polyamide resin, maleate resin, phenol resin, ketone resin and epoxide resin is used as the thermoplastic resin. The use of only one type of resin leads to qualitatively unsatisfactory images in one way or another. Whereas, as has been demonstrated by the inventor of the present application, although a thermoplastic polyamide resin leads to satisfactory flexibility and extensibility and thus to no tears in the transfer, sufficient compatibility with the organic noble metal compounds and flux compounds is not ensured, with the result that decoration defects, in particular insufficient gloss, and/or problems during application, such as in particular inadequate flow, occur. Although maleate resins often lead to bright decorations, these are often also aesthetically too dark; phenol resins often lead to insufficient burn-out and thus to insufficient gloss and/or inadequate adhesion. The document mentioned above does not disclose any binder mixtures for overcoming the problems.
DE-AS 14 21 865 also discloses using media with more than one binder, for example a mixture of a rosin and asphalt resin, in noble metal preparations. A combination of this type, however, leads to decoration defects with transfers, including tears.
A commercially available bright gold preparation for indirect printing on glass and bone china is the product GGP 1230 from the Heraeus Co. Although this product enables the production of tear-free and bright decorations, the dishwasher resistance, that is the adhesion to glass, is regarded as unsatisfactory. Visible damage to the decorations occurs after only 20 wash processes in an automatic dishwasher for glassware. Smooth gold films are not produced on bone china with GGP 1230. The adhesion of decorations to various substrates depends both on the flux system and on the binder system. The composition of the binder system in this preparation cannot be determined in detail from chemical analysis, but amide groups make the presence of a polyamide probable. Another preparation, GGP 1215/Heraeus, contains amide groups and structural elements of rosin; this resin is suitable for porcelain and earthenware, but not for glass.